Charles Spearman (1863–1945) believed we have one general intelligence (often shortened to g) that is at the heart of all our intelligent behavior, from navigating the sea to excelling in school. He granted that people often have special, outstanding abilities. But he noted that those who score high in one area, such as verbal intelligence, typically score higher than average in other areas, such as spatial or reasoning ability. Spearman’s belief stemmed in part from his work with factor analysis, a statistical procedure that identifies clusters of related items.

This idea of a general mental capacity expressed by a single intelligence score was controversial in Spearman’s day, and so it remains. One of Spearman’s early opponents was L. L. Thurstone (1887–1955). Thurstone gave 56 different tests to people and mathematically identified seven clusters of primary mental abilities (word fluency, verbal comprehension, spatial ability, perceptual speed, numerical ability, inductive reasoning, and memory). Thurstone did not rank people on a single scale of general aptitude. But when other investigators studied these profiles, they detected a persistent tendency: Those who excelled in one of the seven clusters generally scored well on the others. So, the investigators concluded, there was still some evidence of a g factor.

We might, then, liken mental abilities to physical abilities: The ability to run fast is distinct from the eye-hand coordination required to throw a ball on target. Yet there remains some tendency for good things to come packaged together—for running speed and throwing accuracy to correlate. So, too, with intelligence. Several distinct abilities tend to cluster together and to correlate enough to define a general intelligence factor. Distinct brain networks enable distinct abilities, with g explained by their coordinated activity (Hampshire et al., 2012).

Satoshi Kanazawa (2004, 2010) argues that general intelligence evolved as a form of intelligence that helps people solve novel (unfamiliar) problems—how to stop a fire from spreading, how to find food during a drought, how to reunite with one’s tribe on the other side of a flooded river. More common problems—such as how to mate or how to read a stranger’s face or how to find your way back to camp—require a different sort of intelligence. Kanazawa asserts that general intelligence scores do correlate with the ability to solve various novel problems (like those found in academic and many vocational situations) but do not correlate much with individuals’ skills in evolutionarily familiar situations—such as marrying and parenting, forming close friendships, and navigating without maps.

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